Field of the Invention
The invention relates to a writing and reading method for a non-volatile memory having a ferroelectric capacitor. The material between the capacitor plates has a hysteresis behavior and the non-volatile memory behavior is based on remanent states that are maintained even without externally applied voltage. In order to read out the stored states, a voltage is applied externally to the capacitor and a sense amplifier evaluates the shifted quantity of charge which is dependent on the originally stored state, destructive reading taking place which makes it necessary to write the information back. Ferroelectric materials used typically include PZT (lead zirconium titanate), PLZT (lanthanum-doped PZT), SBT (strontium bismuth tantalate) or SBTN (niobium-doped SBT). Such materials exhibit changes in the hysteresis curve caused by aging. Measurements on such ferroelectric thin films have shown that the hysteresis curve, that is to say the relationship between polarization and an applied voltage, shifts when a stored state is maintained over a relatively long time. The latter effect is referred to as an "imprint", a static imprint and a dynamic imprint being possible.
In a static imprint, the voltage is not changed at all or is changed only over a relatively long period of time, and, in a dynamic imprint, a transition takes place from a remanence state to saturation and then back to the same remanence state, this typically occurring when the same information item is repeatedly read and written back. In order to achieve reliable detection of the memory states, that is to say a sufficient charge difference during reading, it is possible to enlarge the cell capacitor, but this leads to the chip area being enlarged in a disadvantageous manner.
Published, European Patent Application EP 0 767 464, U.S. Pat. No. 5,262,982 and IEEE Proceedings/VLSI and Computer Peripherals/VLSI and Microelectronics, Application in Intelligent Peripherals and Their Interconnection Networks, May 8-12, 1989, pages 1 to 20 to 1 to 23, disclose ferroelectric memories and an associated reading method in which a reduction of the service life on account of the aging properties of the ferroelectric material is avoided as far as possible, for example by avoiding polarization reversal. What is involved in this case is aging caused by frequent changing of the memory state; in the case of the imprint effect, by contrast, to an extent aging occurs because the changing is too infrequent.